3 ms·

Here's the article that I assume this is referring to: http://arxiv.org/abs/1103.1373 http://arxiv.org/abs/1103.1373 I hate judging based on stupid journalist

Here's the article that I assume this is referring to: http://arxiv.org/abs/1103.1373 http://arxiv.org/abs/1103.1373 I hate judging based on stupid journalist writeups of stuff like this, so I'll actually look through the article, for once. The first part of the abstract: "We construct a simple class of compactified five-dimensional metrics which admits closed timelike curves (CTCs), and derive the resulting CTCs as analytic solutions to the geodesic equations of motion. The associated Einstein tensor satisfies the null, weak, strong and dominant energy conditions; in particular, no negative-energy "tachyonic" matter is required." Translated for the layman: "We completely arbitrarily and quite deliberately construct a geometry that allows normal particles to time travel. It turns out that under this geometry, if a particle time travels, it doesn't have to be abnormal." Let's just say I'm not impressed so far...we'll see how the rest of the article goes...

bermanoid 16y ago
Ok, so I gave this a really quick read-through, and as usual the article way overstates what's been done. I'll try to put this briefly, without too much technical detail. General relativity permits (and we've always known this) solutions that involve closed timelike curves (CTCs). Usually, when we look into these solutions in detail, we find that they require strange sorts of matter to sustain themselves, or that they're unstable. Hawking proposed that the universe in some way bans any geometry that would allow these solutions. The authors here claim that by adding a fifth dimension that's curled up like a circle, and adding a specific sort of curvature to it (the metric contains a periodic cross term between the usual time dimension and the curled dimension), you can get CTCs that are geodesics (i.e. paths that particles will take with no forces on them), and you don't get weird energy conditions or anything like that. Then they go on about the potential consequences if the universe is actually curved up like this, and so on, but don't offer any particularly compelling reason to believe that it is. So basically, they claim to have shown that it's possible for the universe to be like this, and that no condition we currently know of prohibits it. Meh. Could be, if we assume that they're right - it's always, of course, possible that there's some other problem with their solution that wouldn't be covered by the usual energy conditions, I'm not sure, I've been out of the GR circuit for a while so I don't know what people currently consider to be the most reasonable set of constraints that the Real World follows. But even if they're right, there are plenty of solutions to Einstein's equations that could have happened and don't, so there's nothing too revolutionary here, and mentioning a whole lotta shit about branes and the Higgs boson doesn't change that. This is still quite useful theoretical physics to do, mind you, but it's nowhere near as important as the article makes it out to be.

bermanoid 16y ago

Ok, so I gave this a really quick read-through, and as usual the article way overstates what's been done. I'll try to put this briefly, without too much technical detail. General relativity permits (and we've always known this) solutions that involve closed timelike curves (CTCs). Usually, when we look into these solutions in detail, we find that they require strange sorts of matter to sustain themselves, or that they're unstable. Hawking proposed that the universe in some way bans any geometry that would allow these solutions. The authors here claim that by adding a fifth dimension that's curled up like a circle, and adding a specific sort of curvature to it (the metric contains a periodic cross term between the usual time dimension and the curled dimension), you can get CTCs that are geodesics (i.e. paths that particles will take with no forces on them), and you don't get weird energy conditions or anything like that. Then they go on about the potential consequences if the universe is actually curved up like this, and so on, but don't offer any particularly compelling reason to believe that it is. So basically, they claim to have shown that it's possible for the universe to be like this, and that no condition we currently know of prohibits it. Meh. Could be, if we assume that they're right - it's always, of course, possible that there's some other problem with their solution that wouldn't be covered by the usual energy conditions, I'm not sure, I've been out of the GR circuit for a while so I don't know what people currently consider to be the most reasonable set of constraints that the Real World follows. But even if they're right, there are plenty of solutions to Einstein's equations that could have happened and don't, so there's nothing too revolutionary here, and mentioning a whole lotta shit about branes and the Higgs boson doesn't change that. This is still quite useful theoretical physics to do, mind you, but it's nowhere near as important as the article makes it out to be.